Blog
-
Posted: June 22, 2023Read more »
Target flowmeters measure flow by measuring the amount of force exerted by the flowing fluid on a target suspended in the flow stream. The force exerted on the target by the flow is proportional to the pressure drop across the target. Similar to differential pressure flowmeters, Bernoulli’s equation states that the pressure drop across the target (and hence the force exerted on the target) is proportional to the square of the flow rate. Using this relationship, 10 percent of full scale flow produces only 1 percent of the full scale force. At 10 percent of full scale flow, the target flowmeter accuracy is dependent upon the transmitter being accurate over a 100:1 range of force. Transmitter accuracy is typically degraded measuring low forces in its range, so flowmeter accuracy can be similarly degraded. Therefore, this non-linear relationship can have a detrimental effect on the accuracy and turndown of target flowmeters. Remember that of interest is the accuracy of the flow measurement s
-
Posted: June 21, 2023Read more »
When a fluid passes by an object or obstruction, oscillations can occur. Examples of these oscillations in nature include the whistling caused by wind blowing by the branches of trees, the swirls produced downstream of a rock in a rapidly flowing river, and the waving of a flag in the wind. Note that in all of these examples, when the flow is slowed, the oscillations stop. That is, the whistling stops when the wind dies down, the water flows calmly around the rock when the river is not flowing rapidly, and the flag does not wave in a mild breeze.
Fluidic flowmeters are flowmeters that generate oscillations as a result of flow. Vortex shedding flowmeters use a bluff body obstruction, whereas other fluidic flowmeters include designs based upon the Coanda effect and vortex precession. Increasing flow increases the frequency of oscillation. A sensor detects the oscillations and a transmitter generates a flow measurement signal.
-
Posted: June 21, 2023Read more »
Variable area flowmeters measure flow by allowing the flow stream to change the opening within the flowmeter by moving an internal part. When the flow increases, the fluid generates more force and moves the internal part farther.
One variable area flowmeter measures flow in a vertical metering tube by balancing the downward weight of a float with the upward force of the flowing fluid. Spring-opposed float designs allow this type of flowmeter to be installed in horizontal pipes, because the functioning of the float is not dependent upon gravity. These flowmeters can be read locally because their glass or plastic metering tubes have markings that relate the height of the float (that can be seen) with the flow rate of the fluid. Flowmeters with remote signals are typically constructed with metal tubes, and include a transmitter that senses the height of the float to determine fluid flow.
Vane-style variable
-
Posted: June 21, 2023Read more »
Ultrasonic flowmeters use sound waves to determine the velocity of a fluid flowing in a pipe. At no flow conditions, the frequencies of an ultrasonic wave transmitted into a pipe and its reflections from the fluid are the same. Under flowing conditions, the frequency of the reflected wave is different due to the Doppler effect. When the fluid moves faster, the frequency shift increases linearly. The transmitter processes signals from the transmitted wave and its reflections to determine the flow rate.
Transit time ultrasonic flowmeters send and receive ultrasonic waves between transducers in both the upstream and downstream directions in the pipe. At no flow conditions, it takes the same time to travel upstream and downstream between the transducers. Under flowing conditions, the upstream wave will travel slower and take more time than the (faster) downstream wave. When the fluid moves faster, the difference between the upstream and downstream
-
Posted: June 21, 2023Read more »
Turbine flowmeters use the mechanical energy of the fluid to rotate a “pinwheel” (rotor) in the flow stream. Blades on the rotor are angled to transform energy from the flow stream into rotational energy. The rotor shaft spins on bearings. When the fluid moves faster, the rotor spins proportionally faster. Turbine flowmeters now constitute 7% of the world market.
Shaft rotation can be sensed mechanically or by detecting the movement of the blades. Blade movement is often detected magnetically, with each blade or embedded piece of metal generating a pulse. Turbine flowmeter sensors are typically located external to the flowing stream to avoid material of construction constraints that would result if wetted sensors were used. When the fluid moves faster, more pulses are generated. The transmitter processes the pulse signal to determine the flow of the fluid. Transmitters and sensing systems are available to sense flow in both the forward and rev
-
Posted: June 21, 2023Read more »
Thermal flowmeters use the thermal properties of the fluid to measure the flow of a fluid flowing in a pipe or duct. In a typical thermal flowmeter, a measured amount of heat is applied to the heater of the sensor. Some of this heat is lost to the flowing fluid. As flow increases, more heat is lost. The amount of heat lost is sensed using temperature measurement(s) in the sensor. The transmitter uses the heat input and temperature measurements to determine fluid flow. Most thermal flowmeters are used to measure gas flows. Thermal flowmeters represent 2% of global flowmeter sales.
The amount of heat lost from the sensor is dependent upon the sensor design and the thermal properties of the fluid. The thermal properties of the fluid can and do vary with pressure and temperature, however these variations are typically small in most applications. In these applications where the thermal properties of the fluid are known and relatively constant during
-
Posted: June 21, 2023Read more »
Propeller meters are very similar to turbine meters in technology and application. The main difference between the two is in the rotating element and it's suspension in the fluid stream. A propeller is usually made of thick injection molded plastic and faces directly into the flow, suspended from a single bearing assembly. Turbines are thinner and are usually supported on both sides by two, lighter-weight bearing assemblies.
Propeller flowmeters use the mechanical energy of the fluid to rotate a “pinwheel” (rotor) in the flow stream. Blades on the rotor are angled to transform energy from the flow stream into rotational energy. The rotor shaft spins on bearings. When the fluid moves faster, the rotor spins proportionally faster.
Shaft rotation can be sensed mechanically or by detecting the movement of the blades. Blade movement is often detected magnetically, with each blade or embedded piece of met
-
Posted: June 21, 2023Read more »
Positive displacement flowmeter technology is the only flow measurement technology that directly measures the volume of the fluid passing through the flowmeter. Positive displacement flowmeters achieve this by repeatedly entrapping fluid in order to measure its flow. This process can be thought of as repeatedly filling a bucket with fluid before dumping the contents downstream. The number of times that the bucket is filled and emptied is indicative of the flow through the flowmeter. Many positive displacement flowmeter geometries are available.
Entrapment is usually accomplished using rotating parts that form moving seals between each other and/or the flowmeter body. In most designs, the rotating parts have tight tolerances so these seals can prevent fluid from going through the flowmeter without being measured (slippage). In some positive displacement flowmeter designs, bearings are used to support the rotating parts. Rotation can be sensed mechanically
-
Posted: June 21, 2023Read more »
Paddlewheel flowmeters use the mechanical energy of the fluid to rotate a paddlewheel (just like a riverboat) in the flow stream. Paddles on the rotor are inserted into the flow to transform energy from the flow stream into rotational energy. The rotor shaft spins on bearings. When the fluid moves faster, the paddlewheel spins proportionally faster. Shaft rotation can be sensed mechanically or by detecting the movement of the paddles. Paddle movement is often detected magnetically, with each paddle or embedded piece of metal generating a pulse. When the fluid moves faster, more pulses are generated. The transmitter processes the pulse signal to determine the flow of the fluid.
How to Use Paddlewheel Flowmeters
-
Posted: June 21, 2023Read more »
Magnetic flowmeters use Faraday’s Law of Electromagnetic Induction to determine the flow of liquid in a pipe. In a magnetic flowmeter, a magnetic field is generated and channeled into the liquid flowing through the pipe. Following Faraday’s Law, flow of a conductive liquid through the magnetic field will cause a voltage signal to be sensed by electrodes located on the flow tube walls. When the fluid moves faster, more voltage is generated. Faraday’s Law states that the voltage generated is proportional to the movement of the flowing liquid. The electronic transmitter processes the voltage signal to determine liquid flow.
In contrast with many other flowmeter technologies, magnetic flowmeter technology produces signals that are linear with flow. As such, the turndown associated with magnetic flowmeters can approach 20:1 or better without sacrificing accuracy.